The present invention relates to an energy storage device that can enhance the performances of batteries in numerous applications. More specifically, the present invention relates to an iron oxide compound used as the electrode material for supercapacitors.
Supercapacitor is also known as ultracapacitor or electric double layer capacitor. In rigid terms, though there is some distinction among them, they all can store a large quantity of charges up to several thousands farad (F) in compact sizes. Furthermore, they all have high power density ( greater than 1 KW/Kg), high charge-discharge life ( greater than 104 cycles), and high discharge efficiency ( greater than 90%). The high power density of supercapacitor derives from its quick-discharge characteristics in conjunction with large capacity of energy-storage. Such high power density imparts supercapacitors and the like a unique role as the peak-current provider in hand-held electronic devices, portable power tools electrical vehicles (EVs) and automatic actuators.
All primary and secondary batteries are generally used to deliver small currents for lengthy times. This is due to the energy storage of batteries involves bulk oxidation-reduction which is thermodynamically controlled. Some batteries, such as lead-acid batteries, are capable of discharging quickly, delivering an instant large current greater than 100A in applications like the ignition of automobiles. Nevertheless, the batteries can only provide such large output at very short periods and infrequent repetitions, otherwise the batteries will soon be drained or damaged. In addition to miniaturization of the consumer electronics with inevitable shrinkage of batteries, the EVs are in urgent need for reducing oil consumption and air pollution, batteries should work in parallel with supercapacitors to fulfill the power requirements that batteries alone could not offer. In the parallel connection of batteries and supercapacitors, the latter can virtually provide any peak-current required repeatedly. This allows the batteries to discharge at rated currents, and, as a consequence, the use-time and the life-time of batteries are prolonged. The aforementioned effect is called load leveling. In light of no limitation, except voltage (not to exceed the rated value), on the charging mode of device, supercapacitors are a more versatile energy-storage device than battery. Especially in the regenerative braking of EVs, supercapacitors can quickly and safely save the residual kinetic energy of EVs for later use.
The utilization of supercapacitor in the energy-management system of batteries has been validated. However, the present market prices of supercapacitors, as well as their dimensions and specifications, prevent them from general acceptance. Regardless of their merits, supercapacitors must offer an affordable price to be commercially viable. To lower the cost of supercapacitors, an inexpensive and readily made electrode material should be found. The most frequently used electrode materials for supercapacitors include activated carbons and metal oxides. Metal oxides are superior to activated carbons in energy density, conductivity and workability. Oxides of various transition metals including ruthenium, rhodium, iridium, titanium, cobalt, molybdenum, tungsten, vanadium, manganese and nickel are investigated. Ruthenium oxide (RuO2), either in crystalline or amorphous state, and iridium oxide are determined to have a specific capacitance in the range of 100-750 F/g, which is equivalent to or three-time-higher than the value attainable from carbons. Ruthenium is a by-product in the extraction of platinum, hence Ru is rare and expensive. Cost-wise, RuO2 is unsuitable as the electrode material for making supercapacitors for general use. Other compounds such as sulfides, hydrides and nitrides of the aforementioned metals, iron and lead sulfides, as well as molybdenum and tungsten carbides and borides have been tested as the electrode material for electrochemical capacitor. Whereas the energy-storage capability of the above materials is generally low, the cost of the starting metals or precursors for producing the minerals is considerably high, and the fabrication procedures of the metal oxides are costly as well. Clearly, it requires a more economical and easy-of-preparation electrode material than the above substances to solve the cost problem of supercapacitors for wide applications.
As discussed in greater detail below, the present invention provides the most economical material of the existing electrode materials for energy storage through surface adsorption of static charges. The primary object of the present invention is to provide supercapacitors comprising iron oxide as the active material of electrodes of the supercapacitors. Hydrated iron compounds with a chemical composition of FexOyHz, where 1.0xe2x89xa6xxe2x89xa63.0, 0.0xe2x89xa6yxe2x89xa64.0, and 0.0xe2x89xa6zxe2x89xa61.0, can be yielded in a thin film on iron, steel, or other substrates. In conjunction with suitable electrolytes, the electrode materials show capacitance of as high as 0.5 F/cm2 or 320 F/g.
Another object of the present invention is to demonstrate that the black iron (II,III) oxide or magnetite (Fe3O4) is the major component of FexOyHz to be responsible for the high energy-storage capacity of iron oxide. Other form of iron oxide such as FeO, Fe2O3 or FeO(OH) is likely present with the magnetite. Nevertheless, its presence appears to cause no adverse effects.
Yet another object of the present invention is to provide a direct growth of iron oxide film on iron, steel or other substrates. Methods of one-step preparation include chemical oxidation, electrochemical oxidation, dip-coating, and electrophoretic deposition. Among them, chemical oxidation appears to be the most convenient way. As soon as the iron-oxide film is attained, the film-coated substrates are ready to form supercapacitors. Neither binder nor additional electrode-fabricating equipment is required in the present invention. Supercapacitors of the present invention can be prepared in simple procedures and no binder is needed, the present invention can further reduce the preparation cost of supercapacitors.
Still another object of the present invention is to provide iron oxide as the sole or partial ingredient of the electrode materials for supercapacitors. Iron oxide may be used alone, or it may mix with carbons, metal powders or mineral particles to form a composite electrode for supercapacitors. Iron-oxide film may also be formed on a porous support such as Sb-doped SnO2. The aforementioned combinations utilize the low-cost iron oxide to prepare affordable supercapacitors.
The last object of the present invention is to provide an environment-friendly material, iron oxide, for fabricating supercapacitors. Iron oxides are commonly present in numerous ores on earth. Scraps from the spent iron-oxide-electrodes of supercapacitors will cause no harm to the environments. Furthermore, the iron-oxide-electrodes are easy to regenerate and the substrates may be used repeatedly.